condition and function on same predicate-object returns undefined

[bjdmeest]

The yarrrml document below used on the "People (JSON)" example on Matey gives output below-below. I would expect the output of below-below-below. A rml document gets generated though, so I'm not sure whether this is an issue with the yarrrml-parser, or the RMLMapper

prefixes:
  ex: "http://example.com/"
  idlab-fn: http://example.com/idlab/function/
  grel: http://users.ugent.be/~bjdmeest/function/grel.ttl#

mappings:
  person:
    sources:
      - ['data.json~jsonpath', '$.persons[*]']
    s: http://example.com/$(firstname)
    po:
      - [a, foaf:Person]
      - predicates: foaf:firstName
        objects:
          - function: grel:toUpperCase
            parameters:
              - parameter: grel:valueParameter
                value: $(firstname)
        condition:
          function: idlab-fn:stringContainsOtherString
          parameters:
            - [idlab-fn:str, $(firstname)]
            - [idlab-fn:otherStr, "J"]
            - [idlab-fn:delimiter, ""]

gives

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix foaf: <http://xmlns.com/foaf/0.1/>.
@prefix ex: <http://example.com/>.
@prefix idlab-fn: <http://example.com/idlab/function/>.
@prefix grel: <http://users.ugent.be/~bjdmeest/function/grel.ttl#>.

ex:John a foaf:Person;
    foaf:firstName "undefined".
ex:Jane a foaf:Person;
    foaf:firstName "undefined".
ex:Sarah a foaf:Person.

but I would expect

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix foaf: <http://xmlns.com/foaf/0.1/>.
@prefix ex: <http://example.com/>.
@prefix idlab-fn: <http://example.com/idlab/function/>.
@prefix grel: <http://users.ugent.be/~bjdmeest/function/grel.ttl#>.

ex:John a foaf:Person;
    foaf:firstName "JOHN".
ex:Jane a foaf:Person;
    foaf:firstName "JANE".
ex:Sarah a foaf:Person.